Color television systems employing alternating low-frequency components



Marfh 4, 195s W. HAUSZl COLOR TELEVISION SYSTEMS EMPLOYING ALTERNATING LOW-FREQUENCY COMPONENTS Filed March 17. 1951 4 Sheets-Sheet 1 Walter I-Iawz ,Y

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VHS Attorneq March' 4, 1958 COLOR TELEVI HAUSZ SION SYSTEMS EMPLOYING ALTERNATING FREQUENCY /N Mc (RELAT/VE To 'MA/N cAf'fe/ER) 6.0 ma CHAN/VEL w/m-H l Inventor'. Walt@A Hausz,

His Attorney.

COLOR TELEVISION SYSTEMS EMPLOYING ALTERNATING LOW-FREQUENCY COMPONENTS March 4, 195s W HAUS-z I 2,825,753

Filed March 17, 1951 4 Sheets-Sheet 3 \/\/f;lfc"ev` Hausz, by m. bmw@ His Attofh ey.

March 4, 1958 W. HAUsz COLOR TELEVISION SYSTEMS EMPLOYING ALTERNATING LOW-FREQUENCY COMPONENTS f 4 Sheets-Sheet 4 Filed March 17, 1951 LOW P14755 F/LTER una,

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KE v50 #MPL/HER WARE WA Vf wmf/Jr @Mi/MMR Inventor# 'Walter' Hausz,

' b5 M2M His Attorrweg.

United States arent: i

2,825,753 CoLoR TELEvisIoN srsrmrs nineteen-to ALTERNATING Low-FREQUENCY roNENTS Walter Hausz, Syracuse, N. Y., assigner to Electric Company, a corporation of New Your.

Application March 17, 1951, Serial No. 16,2%

13 Claims. (Cl. 17d- 5.2)

My invention relates to new and improved systems for transmitting and receiving television or facsimile images in natural colors, and it has for a main object the transmission and reproduction of a high-quality colored television picture within 'the same technical standards which have been established in the United States for the transmission and reproduction of black-and-white, or monochrome, pictures.

According to current television broadcasting standards in the United States for monochrome picture transmission, the televised scene is sequentially scanned from left to iight and from top to bottom in a series of narrow horizontal lines, in a manner analogous to the way the eye of a reader scans a page of printed material. Each complete scan of the scene to be transmitted, or picture frame, requires the scanning spot at the transmitter or receiver to traverse 525 horizontal scanning lines across the scene within 1,120 second. To reduce dicker, double interlace is employed; i. e., 2621/2 odd lines are first scanned within 1,450 second, constituting one picture eld, and the remaining 2621/2 even lines are scanned during the next picture field to complete the frame. Thus, the horizontal scanning rate is 15,750 lines per second, and the vertical scanning rate is 6'() fields per second. As is well known to those skilled in the art, variousblanking and synchronizing pulses are also inserted at these same rates at the ends of the scanning lines and picture fields. The composite television picture signal, as above described, is modulated upon a picture carrier wave, and any accompanying sound signals are modulated upon a second carrier wave spaced 4.5 mc. above the picture carrier. The two carriers and Lher sideband components are required to be transmitted within a channel having a total bandwidth of 6 mc., approximately 4.75 mc. being devoted to the transmission of the picture signal components. By employing unsymmetrical or vestigial transmission of the picture signal sidebands, a total range of picture signal components up to about 4 mc. can be transmitted. This range of frequencies has been found to be adequate for acceptable resolution of the picture detail in the reproduced image.

As of this date, no nal standards of transmission have yet been established in the United States for the transmission and reception of television images in colors which are comparable to those for monochrome operation. However, due to the tremendous investments which have been made in television transmitters and television broadcast receivers for monochrome operation, it is highly desirable, if not almost essential, that any standards adopted for color television be such as to render as little existing equipment obsolete as possible. To this end, color television transmission should ideally be capable of accomplishment within essentially the same standards as those already established for monochrome transmission, or at least be compatible with present standards. ribat is, the standards for color transmission should be such as to permit a conventional monochrome receiver to reproduce a satisfactory lachend-white in response to re- 2,825,753 etented Mar. 4, V1958 ceipt of a color signal. This immediately creates technical diirlculties, because it is generally agreed that picture signals representative of at least three dillerent color components must be transmitted for production of high-quality color pictures. These are commonly designated as the green, red, and blue picture signals, and they will be so designated for convenience in the following specificatic-n, althc-t n those skilled in the art of colorimetry will understand that the three additive primary color components are usually a green, a red-orange, and a blueviolet and that other color coordinates formed as lineal' combinations of these three primary colors may also be employed.

Thus far, the systems which have been developed for color television may be broadly placed in two classes: (l) those in which the signals representative of the diier.- ent color components are transmitted in a predetermined time sequence by time division multiplex techniques, and (2) those in which the signals representative of the different color components are transmitted simultaneously over different frequency channels.

The rst class includes systems of the so-called held sequential type in which interlaced picture elds are sequentially transmitted in the different colors, of the line sequential type in which interlaced scanning lines are sequentially transmitted in the different colors, and of the "dot sequential type in which the small, individual picture elements are sampled in the dierent colors in a redetermined sequence and then sequentially transmitted. ln all such color television systems, the common problem is presented of transmitting as much picture detail as possible within a transmission channel of predetermined bandwidth. With most of the various sequential systems heretofore proposed, it has been possible to transmit an adequate range of picture frequency components within the band now allotted for monochrome transmission, at the expense of reducing the effective eld repetition rate. This gives rise to dicker, color smearing, and other undesirable edects, to a greater or less degree.

The simultaneous type of transmission permits all three color components to be transmitted at the same repetition rate, but has generally required a much greater bandwidth for the acceptable reproduction of picture detail. ln simultaneous color television systems developed some years ago, bandwidths of from l2 to 16 mc. were employed, but in recent years a considerable reduction in bandwidths has been achieved, without objectionable loss of picture detail, through the use of the mixed highs principle. T his will not be described herein in complete detail, since it is well known to those skilled in the art and described in the literature. See for example the textbook entitled Radio Engineering by F. E. Terman, pages 8544856 (McGraw-Hill, 3rd ed., 1947) and U. S. application, Serial No. 714,750, filed December 7, 1946, by Alda V. Bedford for Simultaneous Multi-colortTelevision, now Patent 2,554,693, granted May 29, 1951.

VeryV briey, the mixed highs system is based 0n the premise that it is not necessary to transmit a full frequency range of video or image signal components for each of the three component colors in order to obtain an image which is satisfactory to the eye. The green signal is transmitted with a substantially full range of components extending up to approximately 4 rnc., and it has mixed with it the higher frequency components of the red and blue signals. The higher frequency components, or highs, of all three signals comprise the mixed highs. Only the lower frequency components, or lows, of the red and blue signals are then transmitted on separate bands, which need not be as wide as the band required for the green signal. At the receiver, equal portions of the mixed highs from the green signal are impressed on each of the threeY cathode ray systems etnp'loyed to reproduce the color images, and only the lows Y are impressed on the respective systems individually.

Y Ytail of the image to appear in shades of gray. The technique is similar to that employed in color printing, in which the fine detail of the image is carried bythe socalled black printerjonly the broaderV detailsV being printed in colors. However, the effect upon the eye of vthe 'observer is not substantially different from that ob tained when `all three complete bands of color compovnerlts are transmitted `and'reproduced separately, `thus allowing a substantial reduction in bandwidth for the same apparent picture detail. l l

Ihe exact Vline of frequency division between the lows andhighs of `Yeach color video o1" image signal hasgbeen found not Vto 'b e particularlycritical!A The eye is mest vQSeIlISitve to the green component, ,andiit is mueh lese sensitive to the red and blue components.V It has been found that only those'freguency components of the red and'blue signals vbellow approximately lV mc. in frequencyv are needed for good color rendition; In fact, the frequency range of the transmitted blue components can be as 'low as .2 mc. or even lower. A good practical average 'for the upper limit ofthe red and blue lows Y might bemin the neighborhood of .5 mc., for example.

Using these 'known techniques to reduce the bandwidths of` theV several component signals as much Vas Y possible, the problem still remains as to how to transmit the three resultant color signals more efficiently; i. e.,

(1*) the l,green lows plus theV mixed highs, (2) the red l lows, and (3) the blue lows.

Within the scope of the problem 1s the attainment of an optically satisfactory balance vof the horizontal and vertical resolution of the scanned scene with reference to the various hues and to VV,the`.br1gl'1tn ess within any given set of imposed standard rates, and Vin particular, broadcasting standards.

It istherefore a primary object of the present invention to'furnish a relatively simple color television sysaccording to current television Vtem, offering increased utilization of a limited'band of operating'frequencies for representing'the chromaticity and brightness details in an optimum optical balance. K According to my invention, the lower frequency components of theredand the blue image signals (red and kblue, lows) corresponding to certain selected scanning hnes or Ygroups of scanning lines are transmitted in eachV spanned field in a predetermined alternating sequence simultaneously with the continuous transmission of Ythe lower frequency components of the green image signals Y (lille green lows) and .the mixed higher frequency compallents of the green, red and blue image signals (the mixedhighs). I n a receiver, the green lows and mixed highs areicontinuously reproduced VasV the corresponding Y components ,of the composite image and the low frequency'red and blue'components are alternately reproduced -in eachY field, the lines of the red and blue images being Yadjustedfas Will be rmore Vparticularly described, to lili out V theirrespective frames. Y fr Y Y The advantages of such a color television system em- Y Vbojdyingmy'inventiom which may bereferred to, in brief,

vas an alternating lows color Vtelevision system, thus become apparent in that simultaneous transmission Vof only two color image signals is required, while as to the timef divided VAred andfblue lows, the advantages of a line sequential, vratlfter than eld sequential system, are pre-V served. Since the human-eye does not Vrequire the same amount v ofcolor informationforeachof the primary proved color television or Vmultiplex facsimile system in which a better balance of colorY and brightness fidelity is achieved, thereby decreasing the time d nring which certain signals are transmitted in accordance with Vtherability of the eye to resolve Vthe colors corresponding to those signals. Y

J'More Specifically, Vit is en O bieet 0f my invention te provide an' improved lcolor ytelevision system in which Y the scene to be transmitted and reproduced is analyzed Y into three main bands of frequency components'whichV eenienty `dene the color characteristics of the scene, and in which the higher frequency components of all bands are transmitted and received simultaneously whereas certain low frequency components are' sampled and transmitted and received sequentially.

For Vadditional objects and advantages, andfora better understanding of my invention, Vattention ris now. di-

rected to the .following detailed description and accom panying drawings. The features of my invention believed to be novel are'ralso particularly Vpointed out in the appended claims.

In the drawings: f

Fig. 1 isa VsimplifiedV representation of a frequency spectrum occupied by V a `color television signal and ac V companying sound signal transmitted in Yaccordance with the principles of Ymy invention;

Fig. 2 is a one-line block `diagram of acolor television Vtransmitter Vembodying my invention for radiatingsignais having the characteristics illustrated in Fig. l;

VFig. 3 is alone-linek block diagram of a color television receiver constructed'to receive and vreproduce the kcolored scene in response to ysignals received from the transmitter of Fig. 2; A i Y l i Fig. 4 is af-conventionalized representation similar@ that of Fig. l of the frequency spectrum of ka ktelevision signaljwhich Vis a modification of that represented by Fig. -5 is anne-line, block diagram Gij-,another form of color television transmitterrembodying my invention v'for radiating Ythe signals'rrepresented in FignigandV colorsof a color .television Vsysternfor satisfactory'reso-- ther, with less blue information than red. By placing lution, a satisfactory optical effect isvpr'oduced with much Y e less red'or blue information than green, and, Vgoing fur- Fig. ,6 is a one-line block diagram of another y form 4of Y color television receiverfor receivingiand V(reproducing ythe colorscene in response to signals received from the transmitterorffpFig. 5.V l Y Y In .the ,Several figures ef the drawings. 'eeriesrending elements have been indicated by Correspondingrefer enceV numerals to :facilitate comparison, landthose ciicuitele ments whichfmay in themselves be entirely conventional and whose details formno part of the present invention Q have been indicated in simplified block :formwith apf' propriate legends. l r

- A systemvincorporatingrimy inventionris illustrated Figs. l to 3, and', the ,following paragraphs describe jin l detail apparatus. for `botl'rtransmitting and receiving color-television signals.- `Theparticular'.equipment shows as a preferred -embodimentasystem -inwhich the' green lows'and Yminedhighs are transmitted -continucuslyand Ythe` red and blue 'lows Vare transmittedrrinjanalternating line sequence on asubcarrier. The Vequipmentasshjown is adapted to,operate'withinthelpresent standard hannel-width andto employ the line and field s canningfre quencies standardized for menochrome vtelevision broadassesseu cestino. As such, it is fully compatible with present monochrome equipment.

The operation of the transmitter and receiver to be described is perhaps more readily understood by reference first to the representation of the frequency spectrum shown in Fig. l. This shows a radiated picture carrier Wave with its side band components, together with the usual modulated sound carrier within a standard 6 megacycle television broadcasting channel. The frequency spacings for the main picture carrier and the sound carriers are substantially in accordance with the present standards of transmission for monochrome signals in the United States, for example, as shown on page S43 of the previously mentioned textbook on Radio Engineering by Terman. This is standard vestigial side-band transmission for the picture carrier and its components. Thus, the green lows are indicated as being transmitted substantially as a double side band around a main carrier spaced L25 me. from the lower edge of the channel while the mixed highs (the green, red and blue high frequency components) occupy a single side band from 9.5 me. to 2.9 mc. using the main carrier as the zero reference frequency. For the transmission of the alternating red and blue lows, a subcarrier is amplitude-modulated alternately by the red and blue low frequency video signal components to produce double side bands. This subcarrier and modulation components are modulated on the main carrier as a single side band lying betfeen the upper limit of the mixed highs side band and the sound carrier. As described in connection with the transmitter of Fig. 2, the subcarrier frequency is 3,45,3l2.5 C. i. S. higher than the main carrier. The sound system and its components are not further described since they are entirely conventional and represent a condition affecting the availability of frequencies within a standard band rather than an aspect of my invention.

Reference is now made to the color television transmitter illustrated schematically in Fig. 2. Since all of the individual circuit components and elements of this transmitter may be conventional and of various forms well known to those skilled in the art, they have been indicated in block form to simplify the drawinff. The main carrier Wave is derived in a conventional manner from a crystal oscillator l and frequency multiplier It is amplitude-modulated by the various components of the composite picture signal in the modulated amplifier 3. The complete modulated carrier wav-e is then further conventionally amplified and also preferably passed through wave shaping filters as indicated by the block 4 before being impressed upon a suitable signal transmission channel by the antenna 5. -he output iilter characteristics are preferably such as to provide vestigial side-band transmission of the main carrier, as will be readily understood by those skilled in the art without detailed explanation. For those interested in further details of such filter designs, reference may be made, for example, to the article beginning at page 115 of the Proceedings of the I. R. E, March i941, or to the rticle beginning at page 301 of the RCA Review lanuary 194i. Y

The three-color picture signals generated in a tricolor camera 6 which may be of any known type adapted to scan a colored scene or object 7 and to deliver three synchronized scannings outputs up to aboutV 3 mc. in frequency, respectively representative of the green, red and blue color components of the scene. The camera 6 may, for example, comprise three separate camera pickup tubes, each provided with an appropriate color riltcr and arranged synchronously to scan the scene in proper optical registry. A tricolor camera of the flying spot type might also be used, such as that described in the article appearing in the Proceedings of the l. R. E, September 1947, at pages 862-870.

The green, red and blue picture or video signals generated in the camera are respectively supplied over conductors 6, 9 and lil to three pairs of high pass and low pass lters. The filters of each pair are designed to have substantially the same cutoif frequency, thereby to divide each picture signal into the lows and highs as previously described. This frequency is not particularly critical and is selected to provide a practical compromise between color and detail in the reproduced picture. It may, for example, lie anywhere between about .2 rnc. and l mc. Thus, assuming that each of the three-color signals occupies a band of 0-3.0 mc., the three high pass or band pass filters 11, 12 and 13 may be designed to have pass bands of .5-3.0 mc. and the low pass filters id, t5 and id may be designed to have pass bands of about 0-.5 mc. The lilters are preferably designed to have cutoff characteristics which are not too abrupt, in order to hold ringing transients to a minimum, for reasons well known to those skilled in the art. They should also be nearly complementary so that the overall band pass characteristics of each pair of lters approximates that of the impressed camera picture signal.

The outputs of the green, red and blue high pass filters il, 12 and 13 are respectively supplied to amplifiers 17, i8 and 19 and from there to suitable adding or added circuits 20 in which they are combined to form the mixed highs of the composite picture signal. Various suitable types of circuits for this purpose are known to the art, a simple type of circuit, for example, consisting of a plurality of amplitier tubes whose anodes are connected together across a common output load impedance but whose indiivdual control grids are energized separately from the input signals. While the adding circuits might also be called mixers, the nomenclature applied is selected to emphasize the fact that the various outputs are added linearly rather than modulated as the term mixing sometimes suggests.

The green lows output from the low pass filter 14 is also supplied to its amplifier 2l and then additionally added to the mixed highs in the adding -circuits 2li as shown. The splitting of the green signal into highs and lows and then recombining them in the adding circuits 20 facilitates independent adjustment of the amplitudes of the several color components in order to obtain the desired color balance in the composite signal and reproduced picture. lt is not essential, however, and the green signal does not necessarily have to be separated into high and low frequency components.

The green lows and mixed highs output of the adding circuits 2li contain all the video information transmitted on the main carrier side bands. The red and blue lows are also added in a manner to be described shortly to complete the picture information. synchronizing pulses and blanking pedestals are also mixed with the composite video signal in the adding circuits 2d in a well known manner, and the composite wave is then supplied to a conventional modulator 22, whose output signal amplitude modulates the carrier Wave in the modulated amplifier 3.

The usual pulse signals required for blanking and synchronizing the camera sweep circuits and for supplying the synchronizing pulses and blanking pedestals to video signal are generated in known manner in a master synchronizing and blanking pulse generator 23. Thus, camera blanking signals are indicated as being supplied to the camera 6 over the three conductors 24, and the synchronizing and blanking pulses are indicated as being supplied to the adding circuits 2li over the conductors 255 and 26 respectively. The master pulse generator 23 also is designed to generate certain additional signals for reasons that will shortly become apparent.

in order to generate the alternating blue and red lows, the red lows and blue lows from the outputs of the low pass filters 15 and 16 are respectively coupled to keyed amplifiers 27 and 2S. These ampliers may be of any suitable types known to the art. For example, one common type comprises a pentode ampliier utilizing the `positiev video signal;

Y Y H7 enten-".hrafteristie keying' ,"lfhv signal tbe amplified is impressed `'ou/ the ontrol gridy and the keying or switching o'fffthe amplifier on and off is' accorrip'lishedV by' means of" rectagular pulses impressed uponthe suppressor grid, which gates the tube by alternately rendering it conducting and non-conducting; Y l

The keyed amplifiers 27 and 2,8 areY renderedV alternately conductive inV order topass video Ysignals to"r their outputs by means of a pair of rectangular waves which have similar shapes but opposite polarities, supplied from a keyingA or switching sequence generator 29 over conductors 30 and 31,- re'spect'ively. The wave Vforms are indieat'ed adjacent the respective conductors 30 and 31Yin subsequent reference. Y

To facilitate line sequence keying of the red low and blue W amplifiers 2,7 and 28, a 15,750 C. P. S. timing Y Ysignal (horizontl'line scaningfrequency) is supplied toV tlie lteying'ggenerator 29 over a conductor -32 from the'master pulse generator 23; Color switching between redrlowsuand the blue lows is provided at the ends of horizontal scanningV lines rin a ypredetermined line sequence. For purposes ofillustrating a preferred embodiment, a desirable line sequenee is two redlines to onegbluerline.V Thus, the square wave patterns adjacent Y conductors and 31 each have their Vopposite polarity amplitude levels in a 2 to 1 time ratio, corresponding toY the respective periods of two and one cycles Vif the 15,750 C; P; S. ,timing signal. YThese square wave signals furnished Vto 'the keyed amplifiers are-ofV opposite polarity,

however, in-orderthat one amplifier is keyed or switched on while the other is switched off. A- 30 C; P.- YS; (frame frequency) overriding signal is furnished to the Ykeying sequence generator from the master pulse-genei'atorZS over conductor 33 in order` to start` the keying sequence Y generator'in correct phase on each frame.-

Y ReferringY still to Fig. V2,V the subcarrier shown in Fig. 1

for the `alternating red and blue lows is derived from a master'oscillator 34. This maybe adjusted,` for example, to'o'perate at a frequency of 6,890,625 C; P. S.; which is thel875th`multiple ofene half the line scanning frequency `(7,875 C. P. 8;). This relationship offers some ofthe advantages of a frequency interlacedsystem, as

described in an application by Robert B. Dome,- Serial No. Y I,

176,405, med July 2s,- 1950, new Parent Nja 2,635,140,

- tions are indicated schematically by the b1ocks.47 andr :granted April 14, 1953, andvassigned'to the assignee of the present invention, yin minimizing interference between the carrier and subcarrier side bandsin easeV the filters do ntwholly separate them. The oscillator -frequency'is halved in a frequency divider 35 and is supplied as the alternating lows subcarrier over conductor 36 to a modulated amplifier 37.V There Vit is amplitude modulated byV the alternating red and blue low frequency videocamponents wvhich'are supplied from the key'amplifiers 27 vand 2S over a common conductor V38. The subcarrier and double 'iside band output signal of the modulated v oscillator 34.1 lby means of a frequency dividing and multiplying chain comprisiwJ frequency divider andY ai frequency multiplier 4l conneetedinY series toV the freV queney divider 3S. f The frequency divider 4i) mafysuit# ably4 have a. ratio of-Y 875 to 1y (which `isfractoredY 5' 5 X15 X7). and Vthe frequency multiplier 41.1 may haveV angsfto i1V ratio-sorbet its outpurwhicu is Vfed to the,

master pulse' generator 23 k over conductor 42 isY the re-V quired 31,500 P. s. frequency (wenn is twie me horizontal synchronizing frequency f 15,750 C. P.- S.,)'.

A reeever adapted torreceive the signals Vradiated by the tasinittel' of Fig. 2'isy Shown n` lthe"Sirlpliiid n une block diagram of Figs The Vnous@input end' of Ythis receiver may be thatof a conventional superl'i'et'-,V

erodyne` television receiver in which Vsignals' received onantenna 46 are amplified, converted to` a lower interine` diate frequency by mixing them with a local oscillator freouency, further amplified, and finally detected `to repro-once the composite television picture signal as de-j livered to the modulator 22 of the transmitter of Fig. 2. 'lo simplify the drawing, all Vthese conventional flinc- The demodulated picture signal is Vimpressed on twov separate filters in parallel.v One of these, a low pass filter 49,- is designed to pass frequencies up to 3 inc., which frequencies include the demodulated green lows and the mixed highs.V The other filter is a baud pass filter 50 designed to pass frequencies from 2.9 me. to

'4.0 mc., which frequencies include the alternating lows subcarrier and the red'and blue lows video signals alternately transmitted in time sequence on the AsubcarrierV sidebands. The output of the low pass filter V49 is ampli'fied by `a conventional'video amplifier 51 and'supplied over a conductor 52 Vto that one of the three electron guns of a tricolor cathode ray picture tube 53 adapted' to produce a green image on 'the viewing screen 54.Y Y

The picture tube 53 may be o'frany suitable knownV Y type, for example, the three-gun tube described iufthe'` Y magazine Radio and Television News, lune 1950,

pages 46, 47 and'118 (and particularlyY shown Vin Fig `lv of that article). Alternatively, it is, of course, possible 4to use three separate'cathodeV ray tubes, each having a uoresceut screenV adapted Yto produce an image lin one; of the desired colors, and to employ an optical systemV Y for superimposing the images for visual observation. Ac-

cordingly, in any vreference herein to the image signals as reproduced on the green, red and blue screens, it is intended that either separate or combined screens may be considered, 'asY the nature of the image reproductionV means is'not part of my invention. Y Y

pThe red and blue low frequency video components arederived in detector 55 which demodulates theV signalV fromV the band pass lter 50 containing the 'alternating Y lows subcarrier and its sidebands. The vred and blue lows are Vthen supplied over a common Yconductor 56 tokeyed amplifiers S7 andY 58. The amplifiers are keyed or switched on in sequence corresponding to the'Nalter-nating lows keying sequence of the transmitted signal to pro- Vvide lines or groups of lines of the red and blue lows respectively in time division sequence..V In order that the mixed highs may also be added to the alternating'lows,

a band pass lter 59 isV connected Vto the output of the Y low pass filter 49 previously mentioned, the filter 59 be-Y ing designed to pass a band of frequencies approximately from 0.5 mc. to 3 mc. which band includes rthe mixed highs of Fig.` 1.

The mixed highs Voutput'signal -is Vsupplied to an amplifier 60 and then furnished tothe keyed `amplifiers?! and 58 over commonfconductor 5 6. Theoutputs of the keyed amplifiers 57 and V58 are connected. Y

respectively by conductors 61 and 62 to those electron guns of the picture tube 53 which are adapted to produce. the red andY blue images on the viewing screen' 54. ltwill be understood, of course, that the greenrred andY blue image responsesV on the respective screens lor .screen` portions of the optical reproducer are suitably ,register edl for color reproduction. The mixed green,red andrblue 'nigh frequency components, or'mixed highs, are edn-iY tinuously transmitted to the green screen and .alternftely to the red and blue screens, so that through .persistence of vision the detail information provided VVby Vthe mixed .highs to the green, red and blue-electron guns blends l.as

grey detail ou `the, viewing screen 54.

aanwas y The synchronizing pulse components of the detected signal in the output signal of the amplifier 51 are separated out in a conventional manner in the synchronizing pulse separator 63 and utilized to synchronize the horizontal and vertical scanning circuits 64 and 65 of the picture tube 53 in a well-known manner.

Control of the keying sequence for the alternating lows is also derived from the output signals of the synchronizing pulse separator 63. Thus, pulses at the horizontal scanning rate frequency of 15,750 C. P. S. are carried over a conductor 66 from an input conductor of the horizontal scanning circuits 64, and 643 cycle pulses are carried over a conductor 67 from an input conductor of the vertical scanning circuits 65 to a keying sequence generator 68. Square wave blanking pulses are supplied from the keying generator 5d to the keyed ampliers 57 and 58 over conductors t@ and 7%, respectively. As indicated by the square wave form sketches shown adjacent conductors 69 and 76, the square waves of the blue and red keyed ampliers are complementary on a time axis, that is, of opposite polarity, to render the amplifiers alternately conductive without a substantial time interval or overlap between the periods of operation of the respective amplifiers. The particular wave shapes shown correspond to the two red lines to one blue line preferred sequence of the transmitter of Fig. 2, and the reasons for the use of this sequence are explained in a following paragraph.

lnasmuch as only a fraction of the scanned lines per frame is transmitted and received in either red or blue, it is necessary to broaden the red and blue lines to prevent objectionable horizontal bar or line structure from appearing in an image reproduced on the viewing screen 54. A very simple method is to simply defocus the red and blue scanning beams of the picture tubes. Use of an elliptical scanning spot, with the major axis ot the ellipse vertically aligned, is a desirable alternative as it avoids or minimizes loss of horizontal resolution. Such spots are readily obtained in picture tubes by such means, for example, as elliptical apertures in the electron guns or by control of the potential applied to vertical deflecting plates. Another means is by the application of a much high frequency deflecting signal to the vertical scanning circuits 65 of the red and blue electron guns of the picture tube. Due to persistence of vision, the rapidly detiected lines appear to be broadened.

In the operation of the system of Figs. 1, 2 and 3, the quality of the reproduced color image is satisfactorily achieved despite the limiting conditions imposed by monochrome broadcasting standards. Thus, broadening of the red and blue lines is employed to ll the lred and blue frames and since the red and blue lines are substantially line interlaced rather than eld interlaced (only single lines or small numbers of lines compared to the total number of lines per eld are selected for each sequence) iiicker is minimized. However, another problem, line crawl, and its solution remain to be described.

Use of a simple one `red line to one blue line keying sequence, for example, results in the production of an optical phenomenon known as line crawl in the viewed image, unless additional means are utilized to avoid it. Due to the fact that an integral number of two-line sequences is not Iprovided in the standard 525-line frame, the red and blue bars or lines are respectively vertically displaced by one line in successive fields of a frame, thus appearing to crawl down theyrectangular scanning pattern, or raster. With a preferred keying sequence, such as that previously mentioned, where each complete keying group consists of two lines of red lows and one line of blue lows, or a total of three scanning lines, an integral number of complete groups is scanned in each picture frame when employing conventional 525-line double interlace. Under such conditions, the color distribution produced in a complete picture frame (two consecutive interlaced fields) is entirely symmetrical in its' distribution of the color groups so that no apparent crawl results. Following the general rule that the sum of the` number of red and blue lines in any one group shall be divisible in the number of lines per frame without remainder, a preferred group of scanning sequences in 525 lines scanning is seen to be those having either 3, 5 or 7 lines per group. These may be divided between the red and blue lines in any desired manner but since less i11- formation or resolution of the blue components than of the `red (just as less is needed for the red than for the green) is required to meet the requirements of the human eye, and since the red phosphors are usually less satisfactory than the blue phosphors used in the uorescent screens of present receiver picture tubes, it is usually more practical to select distributions which favor the red signal. Preferred combinations, 'with these limitations in mind, are two red lines and one blue line, three red lines and two blue lines, and four red lines with three blue lines. With any of the above combinations, the resultant color image is free from crawl. At the same time, since each partial color component of the scanned scene appears in each field, there is no objectionable dicker.

yModifications of the effective widths of the scanning beams of the red and blue tubes as previously mentioned seem desirable to prevent an undesirable line or bar structure in the reproduced pattern. Accordingly, with the above combinations, the respective beams preferably are broadened to the width of the number of colored lines in the keying group for filling out each red and blue colored frame without unequal overlap in successive frames. For example, when a two to one color switching ratio is employed, ea-ch red beam and each blue beam must have three times its normal width. Since vertical resolution is decreased as the line broadening ratio is increased, the three-line combination is preferred to the 5 or 7 line combinations.

A second system embodying my invention is illustrated by Figs. 4, 5 and 6 and described in the following paragraphs.

Fig. 4 is another -conventionalized representation of a composite television signal similar to that of Fig. 1 but illustrating some alternative details. In this modification, the -green lows and the mixed highs are modulated upon a main carrier spaced 1.25 mc. from the lower edge of a standard 6 mc. channel, the lows extending for approximately 0.7 mc. on either side of the carrier and the mixed highs extending from approximately 0.7 mc. to 3.2 mc. above the carrier. The alternating red and blue low frequency components are modulated upon a subcarrier 3,898,125 cycles above the main carrier with a full side band on the low frequency side of the subcarrier and vestigial side band on the high side. The sound carrier is not further described inasmuch as it is entirely conventional. One diierence between this signal and that of Fig. 1 is the use of vestigial side band transmission for the alternating lows subcarrier to enlarge the range of frequencies available for both the lows and the mixed highs.

Fig. 5 is a block diagram of a suitable color television transmitter for radiating the signal of Fig. 4. Many of the component circuits, though, are the same as those previously described with reference to the transmitter of Fig. 2 and are therefore indicated by corresponding reference numerals without further description. Those elements of Fig. 5 which are not identical with those of Fig. 2 but Whose functions are the same are also indicated by corresponding reference numerals with the sufiix a added.

`In the transmitter of Fig. 5, the green carrier is produced and modulated in substantially the same manner as previously described. The camera 6a produces a video signal having an upper frequency not less than 3.2 mc., but the mode of filtering the three-color signal components and of separating and of adding togelther the mixed highs is slightly modified with respect widely applied. For example, still using the standard 525-line frame with two interlaced fields for the green signal and mixed highs, the red and blue signals may be time divided as desired in each field, and the red land blue line scanning rates are adjusted so that the raster is scanned during each field in the time allotted. This may be termed a split-field alternating sequence, since the red and blue information alternates at a fraction of the field rate rather than at a multiple of a line rate. it is still compatible with present monochrome equipment, since a monochrome receiver receiving the standard line-frequency green and mixed-highs signals on the main carrier for yblack and white reproduction is unaffected by the alternating red and blue lows, regardless of their respective scanning rates.

In summary, it is apparent that my method of providing an improved color television system through the principle of alternating lows combines desirable features of both sim-ultaneous and sequential systems. Full advantage has been taken of the prior art techniques in the use of mixed highs, inasmuch as it has been apparent that only a small part of the limited 6 mc. bandwidth need necessarily be devoted to colors. Moreover, while utilizing the monochrome broadcasting standards, the more or less wasteful use of the full monochrome vertical resolution for color information is avoided, and m-ore bandwidth is made available for the detail information which is satisfactorily and economically provided in monochrome by the mixed highs. No change in monochrome field and line rates is necessary to provide a composite color image without objectionable flicker and crawl and objectionable line structure have been avoided. While, of course, m-y invention may be utilized for more effective use of any given bandwidth and according to other broadcasting standards than those presently established in the United States for black and white television, embodiments described have illustrated a compatible system by which present receiving equipment may receive a satisfactory monochrome image from the green lows and mixed highs information.

While l have shown and described various specific embodiments of my invention, and certain modifications thereof, it will, of course, be understood by those skilled in the art that other modifications may be made without departing from the principles of the invention. l, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. in a multiplex television system, a plurality of camera means for synchronously scanning a scene at predetermined line and field scanning frequencies and for developing three partial picture signal frequency bands, each corresponding to a different optical characteristic of said scene, means for transmitting one of said frequency bands, keying means for alternately selecting timeinterlaced line groups of the other two of said frequency bands during each scanning field, and m-eans for transmitting said time-interlaced bands simultaneously with said one of said groups.

2. ln a multiplex facsimile system, a plurality of camera means for synchronously scanning a scene at predetermined line and field scanning frequencies and for developing at .east two partial picture signals, each corresponding to a different optical characteristic of said scene, frequency-selective means for subdividing each said signal into high frequency and low-frequency bands, means for combining and transmitting the portions of said signals within said high-frequency bands, keying means for alternately selecting a plurality of groups of integral lines of both said signals within each of said lowrequency bands during each scanning field, successive fields being relatively displaced in said camera means to provide symmetrical registry of said groups of lines in successive fields, and means for transmitting fields of said alternately selected line groups.

3. A tri-color television transmission system comprising tri-color cam-era means for scanning a scene at predetermined line and field scanning frequencies and for developing three corresponding partial picture signals, each said signal extending over a band of video frequencies respectively corresponding to a first, second and third primary color component of said scene, frequency-selective means for subdividing at least the second and third color bands into similar complementary high-frequency and low-frequency sub-bands, means for adding said highfrequency sub-bands and said first color band to form a first composite picture signal, keying means for alternately selecting a plurality of integral line groups each comprising the signals within said second and third lowfrequency sub-bands in alternating sequence during each scanning field, means for combining said selected line groups to form a second composite picture signal and means for simultaneously transmitting said first and second composite signals.

4. A color television transmission system comprising tri-color cam-era means for synchronously etecting double-interlaced scanning of a scene at predetermined line and eld scanning frequencies and for developing three corresponding partial picture signals, each said signal extending over a band of video frequencies respectively corresponding to green, red and blue primary color elements of said scene, frequency-selective means for subdividing the red and blue bands into similar, complementary high-frequency and low-frequency sub bands, means for adding components of each of these signals lying within said red and blue high-frequency sub-bands and within the green band to form a first composite picture signal, keying means for alternately selecting timeinterlaced portions of the components of each of these signals lying within said red and blue low-frequency subbands, each said portion corresponding to an integral number of scanning lines substantially less than the num- F- ber of lines in a picture field, means for combining said selected portions to form a second composite picture signal, and means for simultaneously transmitting said composite signals.

5. A multiplex facsimile system comprising means for scanning a scene and for concurrently developing three image signals each corresponding to a different optical characteristic of said scene, frequency selective means for subdividing at least two of said signals into two substantially mutually exclusive bands of relatively low-frequency and high-frequency components, means for combining signal components of said two high-frequency bands and of said third signal to form a first composite picture signal, keying means for alternately selecting time-interlaced portions of the signals within said other two low-frequency bands in synchronism with the scanning of said scene, means for combining said selected portions to form a second composite picture signal, means for simultaneously transmitting and receiving said two picture signals, means for simultaneously transmitting and receiving a keying signal in synchronism with the operation of said keying means, means utilizing said received picture and keying signals to reproduce said composite picture signals, and a plurality of cathode ray scanning means utilizing said reproduced signals to produce a composite picture portraying said scene,

6. A multiplex color television transmitter comprising a plurality of camera means for synchronously effecting odd-line double-interlaced field scanning of a scene at predetermined line and Field scanning frequencies and for developing at least three partial picture signals, each said signal extending over a band of video frequencies corresponding to a dierent color characteristic of said scene, frequency-selective means for subdividing at least two of said signals into similar, complementary high-l frequency and low-frequency sub-bands, means for al- Vcomposite. picture signal, means for cyclically displacing the positions of the Yinterlaced fields of said two signals `relative Vto each other in said camera means to provide symmetrical registry of the lines of two signals Within said/two said low-frequency sub-bandsQand means for simultaneously transmitting'said two composite .picture signals. Y Y Y 7. A system for multiplexing three corresponding partial .picture television signals, each signal extending over a band of video frequencies respectively corresponding to a first, second and third component color oiga scene, comprising means for v."lividi'ng at least the second and thirdcolor bands into substantially mutually exclusive high-frequency and low-frequency sub-hands, means for adding said high-frequency'sub-bands and said first color band to form a iirst composite picture signal, keying means -for alternatelyrselecting portions of said second Vand third low-frequenc'y'subV-bands in an alternating sequence during each scanning field, means for combining said selected portions to form a second composit epicture signal, Yand meansY for simultaneously transmitting said first and second composite signals.

y8. A multiplex color Vtelevision receiver adapted for operation with the transmitter of claim `4,comprising means for/separately detecting and reproducing each of said Vcomposite picture signals, Ythree cathode-ray line Yscanning means having intensity control electrodes and ray Vdeiiecting means, Vsaid scanning means Veah ,being arranged toproduce by successive `lines of each field a fluorescent image in one of Ysaid green,rrcd and blue primary colors,fm eans for synchronizing the scanning of said three cathode ray means in unison withV the scanning-of said scene, means V'for impressingsaid iirstrcomposite 'signal ont-he control electrode of at least said green scanning means, and means for alternately im'- pressing said, second composite signal on the jcontrolY electrodes of saidred and blue scanning'means in syrichronism withY the transmission of the red andblue portions of said second signal.

9. -A multiplex color television receiver yadapted for operation with the transmitter of claimV 4,Y Vcomprising means for separately detecting and reproducing eachl of said composite picture signals, three cathode-ray line scanning means having :intensity control electrodes and ray defecting means, vsaid scanningmeans each being arranged to produce by successive lines of each field va fluorescent image in one of said green, red and blue primary colors, means forlsynchronzing' the scanning Vof said three cathode ray ,meansV in unisonV with the-scanning of said scene, means forV impressing said first composit'e signal on the control electrode o'f atleast said greenV scanning means,'means` for alternately impressing! said' second .composite signal on the control electrodes of said red and blueV scanning means in synchronism with the transmission ofthe red and blue'iportions of Vsaid second signal, and means for broadening the linesY producing the red and blue images. Y Y Y l0. A color television'transmitter comprising Ytri-'color camera means for synchronously effecting double-inten V'laced scanning of a scene `at predetermined line and field scanning frequencies and for developingVV three corre; spending partial picture signals, each said signal extend' 'ngovcr -a band of Video frequencies respectively correspondingy to three elemental colors of said scene, frequency-selective rmeans for subdividing two lof saidrv sig` nals into similar complementary high-frequency and low-Y frequency sub-bands, Ameans for adding components of i' eaclrofthe sgnalslying within Vsaid highfrequency subhands to signal components lying within said third band-V v of at least a' first one of said Yscanning means, gatingY Y meansrcontrolled byloneof said received signals for al-V Vcathode-ray line scanning meansrrhaving Vintensity control electrodes andrav deflecting',means,y said Yscanning to forrrnaifirst `composite picture signal, keying meansv for lalternately selecting time-interlaced portions of'the` comnoents'of each of the signals Alying within saidlowfrequency sub-bands'each said portion comprising sub` stantiallvless than one picture field'and corresponding scanning frequencies and for developing three corre-V spending partial picture signals, each said signal extend-Y ing over a band of video frequencies Yrespectively corresponding to three elemental colors of said scene, frequency-selective means for suhdividing two Vof said signals into similar 'complementary high-frequency and lowfY frequency sub-bands, means for adding components of each of the signals lying within said high-frequency subbands to signal components lying within said third band to form a first composite picture signal, keyingV means for alternately selecting time-interlaced portions ofthe components of each ofthe signals lying within said lowfrequency sub=bands, each Vsaid portionY comprising sub-A stantially less than one picture field and correspendingV to an integral multiple, i'l'lcluding unity, of one scanningY line, means for combining said selected portions Yto form a second compositeY picture signal, means for simultaneously transmitting and receiving said two composite pic-V ture signals, means for detecting and reproducing said composite signals, three cathode-ray line scanning means having intensity Ycontrol electrodes and' ray deflecting means, said scanning` means each' being arranged to produce by successive lines of each field a Vfluorescent image in voneof said elemental colorsymeans controlled by said signals for synchronizing the scanning of said three cathode-ray means in unison with the scanning of said scene, means for impressing said first signalronthe control electrode of at least a firsty one of said scanning means, gating means controlled by one of said received* signals for alternately impressing said Vsecond signal on the control electrodes of the 'second and third'ones of` said scanning means in synchronism with the alternations of said keying means, and means to broaden the widths. of the lines Ypro'ducedfby said second and third ones of saidmeansr so that the areas of their resultant fluorescent images are `each comparable to the area of Vthe image produced by Vsaid first one of said scanning means. i Y Y l2. lA multiplex color television receiver adapted for operation with the transmitter of claim 4 comprising Vmeans for Adetecting and reproducing said compositev sig-VV nals, threefcathode-ray line scanning means having *iii-V tensity control electrodes and ray deflecting means, said scanning meansV each being arranged to producerby suc- Y cessive lines of each field a fluorescent image in one of said elemental colors, means controlled by ,said signals Vfor synchronizing the scanning of said fthree cathode-ray means in unison with the Vscanning, of said scene, means for impressing said first signal on the control electrode ternatelyimpressing said second signal, on the .control electrodes of the second andthird ones of said scanning Y means in synchronism with the alternations of saidfkeying meanrs,and meansV to broaden the widths of Vthe 'linesV produced by said Ysecond and third ones of said scanning means as Ycompared toithe widths of the lines by the first one of said scanningmseans. j

' i3. A color television receiver adapted for Yoperationwith the transmitter of claimvf comprisingemeans for detecting and reproducing Asaid composite signals, three produced.:

17 means each being arranged to produce by successive lines of each iield a fluorescent image in one of said elemental colors, means controlled by said signals for synchronizing the scanning of said three cathode-ray means in unison with the scanning of said scene, means for impressing said rst signal on the control electrode of at least a rst one of said scanning means, gating means controlled by one of said received signals for alternately impressing said second signal on the control electrodes of the second and third ones of said scanning means in synchronism with the alternations of said keying means, and additional keying means controlled by one of said signals for cyclically displacing the uorescent images produced by the second and third ones of said scanning References Cited in the le of this patent UNITED STATES PATENTS 2,333,969 Alexanderson Nov. 9, 1943 2,335,180 Goldsmith Nov. 23, 1943 2,535,552 Schroeder Dec. 26, 1950 2,554,693 Bedford May 29, 1951 2,567,040 Sziklai Sept. 4, 1951 2,677,720 Bedford May 4, 1954 10 2,773,929 Loughlin Dec. ll, 1956 OTHER REFERENCES A Six-Megacycle Compatible High-Definition Color Television System, Television, Volume VI, 1949-1950,

means in synchronism with the displacement of the cor- 15 pages 270-290, published by RCA Review.

responding interlaced fields at the transmitter. 

